(1) Field of the Invention
The present invention relates to a plasma generating apparatus and a method for controlling plasma-generating high frequency power to be supplied to plasma, for use in a producing process of a thin film component, a particle beam source or an analysis apparatus, and more particularly, to a technique for appropriately and easily control the plasma density.
(2) Description of the Related Art
In recent years, the use of plasma is increased. In a producing process of a thin film component, using high-frequency plasma generated by high-frequency power (high-frequency electric power) in a range from a RF frequency band of about 10 MHz to a micro frequency band of 2.45 GHz, etching process or CVD (chemical-vapor deposition) are conducted. In such a plasma application technique, it is extremely important for conducting appropriate process to correctly control the plasma density.
The plasma density in a conventional plasma generating apparatus is controlled by controlling plasma-generating high frequency power in the following manner. The plasma-generating high frequency power is supplied from a high frequency electric source to plasma through an impedance matching device. A portion of the high frequency power that was not absorbed by the plasma and reflected and returned to the high frequency electric source side, i.e., reflected power is detected. The matching state of the impedance between the electric source side and the plasma is adjusted by always automatically controlling the impedance matching device such that the reflected power becomes minimum, thereby stabilizing the plasma density to keep the processing condition constant.
However, there is a problem that the conventional control of the plasma density is not appropriate. The plasma density is substantially proportional to net electric power applied to plasma, but even if the impedance matching device is automatically adjusted such that the reflected power of the high frequency power becomes minimum, only high frequency power including loss between the impedance matching device and an ignition electrode (discharge antenna) becomes constant, net electric power applied to the plasma does not always become constant. Therefore, there is an unavoidable limit in the conventional control method of the plasma density in which the plasma-generating high frequency power is controlled based on the reflection amount of high frequency power that is not absorbed by plasma and reflected and returned.
To solve the above problem, it is considered that the plasma density is controlled based on the actually measured plasma density. In the case of typical plasma comprising monovalent positive ion and electron, the positive ion density and the electron density are substantially equal to each other due to the properties particular to plasma that electrically neutral state is maintained, the electron density is generally called as plasma density. That is, the electron density in the plasma is substantially equal to the plasma density. Conventionally, as a method for measuring the electron density in plasma, there is an electron beam irradiation type plasma vibration probe which was developed relatively recently, in addition to a Langmuir/probe method and a microwave interference measuring method. It can be considered that electron density in plasma is measured by these method, the plasma-generating high frequency power is controlled based on the measured electron density.
The Langmuir/probe method is a method in which a metal probe is directly exposed in plasma an in this state, direct current bias voltage, or direct current bias voltage on which high-frequency voltage is superposed is applied to the metal probe, and based on the current value flowing through at that time, electron density is measured. The microwave interference measuring method is a method in which a chamber for generating plasma is provided with windows which are opposed to each other with plasma positioned therebetween, microwave (e.g., single color laser light) is radiated to the plasma through one of the windows, and the microwave ejected from the other window is detected, and electron density is obtained based on phase contrast between the radiated microwave and ejected microwave. The electron beam irradiation type plasma vibration method is a method in which a hot filament is placed in a chamber, and based on frequency of plasma oscillations generated when electron beam is irradiated to the plasma from the hot filament, electron density is obtained.
However, the Langmuir/probe method has a problem that the measuring can not be continued for a long time. This is because that stains comprising insulative films are adhered on a measured metal probe within a short time, the current value flowing through the metal probe is varied, and accurate measurement can not be continued soon. In order to remove the stains adhered on the metal probe surface, a method in which negative bias voltage is applied to the metal probe to carry out sputter-removing method using ion, and a method in which the metal probe is allow to glow to evaporate and remove the stains have been attempted, but the effect is poor, and the problem is not solved by these methods.
Further, the microwave interference measuring method has a problem that the measurement can not be conducted easily. This is because a large-scale and expensive apparatus and adjustment of microwave transmission path are necessary, the phase contrast between the radiated microwave and ejected microwave is small and thus, it is difficult to measure precisely.
Furthermore, in the case of the electron beam irradiation type plasma vibration probe method, in addition to anxiety of plasma atmosphere contamination due to tungsten which is evaporated from the hot filament, there is a problem of anxiety of interruption of measurement caused by break of hot filament. Especially in the case of plasma using oxygen or chlorofluorocarbons gas, the hot filament is easily cut or broken, and it is necessary to frequently exchange the filament, it can not be said that this is practical.